In mid November, winter crane flies (Diptera) begin dancing while hundreds of yellow-legged meadowhawk dragonflies (Sympetrum vicinum) continue to fly over local ponds in the northeastern states. Migration is a seasonal activity. Animals of all kinds move north in the spring and south in the autumn. It is not unusual to find the first of the winter migrants and the last of the summer animals together in the middle of autumn.
Caribou, birds, fish, and insects each follow specific pathways while migrating. The ranges of many plants and animals have expanded northward for many decades. In these movements might be clues as to why animals undergo their seasonal migrations.
Insect Wanderings
Juvenile insects of most species often wander long distances from their birthplaces until they settle in a breeding location. While breeding, animals have known food resources, but many insects migrate either before or while breeding and move across areas with unknown resources. They often form large aggregations and follow neighbors to obtain information on scattered food locations.
Before the Pleistocene, pollen indicates even the poles of the planet basked in subtropical warmth and seasonal temperature changes were minimal. In such a world, animals had little need to move from place to place to find food and most movements were of juveniles dispersing.
Enter Ice Age Glaciations
Beginning about seventy thousand years ago, much of the northern continents were covered with ice sheets that rivaled those of present day Antarctica. This was the last of a series of four major glaciation events during which ice accumulations up to two miles thick lay over the northern regions of the world making these lands uninhabitable for almost all life. River valleys were carved and mountains eroded by these massive ice sheets which only fully retreated from New England about eight thousand years ago.
How Ice Affected Animals and Plants
As plants became buried under permanent ice accretions, animals that were once able to breed in the far north had to move to more southern climates to find suitable breeding grounds. During the winters, these same animals would have to move even farther south to avoid freezing.
These southward movements in response to ice sheets and cold winters also provided direction to the wanderings of juveniles and adults of many kinds of animals. Thus changes in climate forced individuals to make southward movements and autumnal migration patterns became established. Paths were often moderated by geological barriers and prevailing wind directions. Thus, insects aggregated during their migrations - sometimes in huge numbers.
Other Factors Triggering Migration
The arrival of monsoon rains in India and South-east Asia are followed by large scale migrations of dragonflies and other insects that sometimes fly over 2100 miles (3500 km) of open ocean, making only a few stops on oceanic islands before they reach Africa.
When food supplies are plentiful and population levels rise, several species of desert grasshoppers aggregate in huge numbers, alter their coloration (after which they are called migratory locusts), and begin migrating as they deplete their food supply. They repeat this behavior, wandering from location to location where they strip the vegetation from the land and leave behind barren wastes.
Life Changes During Migration
Insects usually do not live long enough to complete the entire migratory circuit, and time their migrations to an annual cyclic clock. Some, like the monarch butterfly (Danaus plexippus), stop breeding when they begin to migrate. Others, like dragonflies and migratory locusts, breed wherever there is appropriate habitat. Migrating insects often fly on favorable winds following cold fronts while biological clocks and compasses control the direction, distance, and timing of migrations.
Stopping to rest and feed when winds are less favorable and temperatures are mild, migrants are oriented to a specific suite of climactic conditions that, once found, become their winter home. The mechanisms by which these conditions were established and coded for in the behaviors of animals may never be fully understood.
Post-Migratory Behaviors
When insects finish migrating - often on winter grounds, dragonflies disperse and establish non-exclusive winter territories within local populations of related species. Migratory locusts retain their exclusive groups and move through local areas - sometimes consuming all available foods in the area. Millions of monarch butterflies gather in suitable habitat and enter a state of torpor that is broken by the return of warm, moist days.
The Effect of the Ice Melting
As the ice receded over the period of twelve to eight thousand years ago, plants at the northern edge of their range began to expand northward. Insects followed the plants, and predators followed the insects. Migrations became important survival strategies in the life histories of these animals.
As daylength increases in the spring and new plant growth is established along the pathway, migrants initiate a reverse migration; as daylength increased, they resume breeding and die. The young grow, metamorphose, and continue the migration - again allowing topography and weather patterns to direct them. This restlessness ultimately becomes depressed as summer temperatures and photoperiod return.
The urge to migrate is not found in all insects. Many reproduce and die before cold weather or drought sets in. Those that do migrate have genetic triggers that respond to photoperiod and weather patterns. For those species that do migrate, individuals that remain behind normally die when adverse conditions develop. If the climate changes so that there is no need to migrate, those individuals that do not migrate would have an advantage over their migrating relatives and migratory behavior may gradually be phased out.
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